Preheat grill for direct cooking to medium heat. Fold a 36-by-18-inch piece of heavy foil in half to make an 18-inch square.Scrub potatoes thoroughly with a brush; pat dry. Cut potatoes into uniform 1-inch pieces. Place in center of foil.In a small bowl combine butter, garlic, seasoning salt and rosemary. Drizzle over potatoes. Bring up 2 opposite edges of foil; seal with a double fold. Fold remaining edges to completely enclose potatoes, leaving space for steam to build.Grill potato packet for 35 to 45 minutes or until potatoes are tender, turning packet several times during cooking. Remove packet from grill. Carefully open packet.